Urban Forests: Environmental Health Values and Risks

Forests accompany the cities we build. There are an estimated 5.5 billion urban trees in the United States. Globally, about 25 percent of urban land is covered by tree canopy. This study examines urban forests as a policy tool for air pollution mitigation. We study an afforestation program in the city of Beijing, which planted a total of 2 million mu of greenery – roughly the size of Los Angeles – across the city over a decade. We conduct a remote-sensing audit of the program, finding that it contributes to a substantial greening up of the city. This causes significant downwind air quality improvement, reducing average `PM_2.5` concentration at city population hubs by 4.2 percent. Rapid vegetation growth, however, led to a 7.4 percent increase in pollen exposure. Analysis of medical claims data shows aeroallergens triggered emergency room visits, mirroring well-documented industrial pollution effects though less severe. We offer insight on managing urban forests’ health risks, identifying harmful pollen species and susceptible population subgroups.Institutional subscribers to the NBER working paper series, and residents of developing countries may download this paper without additional charge at www.nber.org.